Bale will join the suspended Cristiano Ronaldo on the sidelines for Wednesday's match as Real seek victory to secure passage to the quarter-finals after a 0-0 draw in the away leg.
"Bale has taken a knock to the calf," Ancelotti said. "We will see how it evolves in the coming hours but I do not think it is a problem for Sunday."
Real, five points behind Barcelona and Atletico Madrid in La Liga, travel to Valencia at the weekend.
Real are coming off a 2-2 draw at Osasuna which saw them lose further ground on their rivals, but Ancelotti is not worried.
"I don't think Osasuna was a bad game," he said. "The first 20 minutes were the best by the team this season. We must continue and be confident."